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Old 20th Jul 2020, 9:00 am   #21
Radio Wrangler
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Default Re: Stereo 50w 807 PP amp.

Andy, look up 'star quad cable'

https://en.wikipedia.org/wiki/Star_quad_cable

it's just a clever way of distributing currents in a twisted quad of wires.

So Joe aligns the slots in screw heads carefully..... But I didn't know you had ley lines in Oz to align them with?

David
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Old 20th Jul 2020, 10:08 am   #22
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Default Re: Stereo 50w 807 PP amp.

Im sure they are there David ;-), somebody must have seen them.
ERHHH and I use Allen head screws Unbrako brand which are usually hi tensile grade 10.9s as a minimum. Now I take the picture and they are branded Brighton Best ho hum. Anyway Uluru, the former Ayers rock seems to be aligned.
http://www.kanaga.tv/story/ley-lines.html


Cheers for now

Joe
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Old 21st Jul 2020, 7:52 am   #23
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Default Re: Stereo 50w 807 PP amp.

Will do David, is it expensive? It looks a good way of dealing with heater cabling. Don't go as far as aligning bolt slots with anything, including lay lines or using vintage NOS bolts but like to keep things neat. One visual aspect that is a bit of a nightmare is finding the right knobs and another is labelling and tarting up. By the time I get to this point where the amps working my enthusiasm is waning fast, I want to skip off into the sunset like fickle Bobby onto the next project.

Did some extensive testing yesterday to try and find at what frequency it will resonate in an effort to get my head round poles and stabilty. I was testing with a purely capacitive load, EG 0.1u. this is the first time I've tried this and it was illuminating.

On the LF side of things you can see where the OPT core saturates causing immense distortion, it also starts to go unstable around 30hz. On the top end I ran out of F, my HP3903B only goes up to 50khz, I found no resonance up this end, but it's there, so I need to get the old Marconi RF boat anchor out.

If I've got this testing with a capacitive load right, you test @ various OP levels starting at 0dB with no NFB applied, low capacitance load looking for where distortion increases or there's a signal peak. This is done at various decreasing levels noting the same as we go and maybe increasing C.

The source I referred to also mentioned testing without a load - http://www.turneraudio.com.au/amplif...ncy-tests.html his text isn't 100% clear, like Joe he's an Aussie and has a Australian's way of torturing English, so one needs a translation. Like most audio types he also has his inbuilt biases but seems to mostly know his stuff.

Anyway, the point of all this is to make the amp unconditionally stable with any load it's likely to see, this isn't easy to do and most amp builders/designers don't bother with this side of amp design, AFAIK that is.

One aspect I'm slightly struggling with is low frequency, low pass shelving networks needed to reduce gain at LF so the amp is stable as mentioned. I'm fine with HPF, use 1/ (2*pi * R * F) cap to ground or the lowest Z path to leak the frequency we're after to ground or in conjunction with an R, so cap in series, R in parallel with load to form a HPF. Implementing a LPF is alluding me at present, but am doing some reading up on the subject. All these are added to the existing amp design.

One other approach to amp stability mentioned in Radio Designers and the GEC An Approach to AF Amplifier Design is to chose the right value of coupling caps for each stage, this being part of the design. There are different approaches for 2 stage and 3 stage design from what I've read so far, making some stages flat and others attenuate degenerative frequencies, still getting my head round this, it taking into account ra, gain, inter electrode C etc. Mostly I try a value after some calculation the test. The testing can be very tiring it taking a good 30 minutes for one frequency sweep, i do learn alot though.

Andy.
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Old 21st Jul 2020, 9:52 am   #24
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Default Re: Stereo 50w 807 PP amp.

Star quad is just a name for a layout of conductors in a twist. Commercial cables are available, but the gauges are intended for small signals like microphone cables.

What MM was suggesting was stealing the idea for your heater wiring twisted bundles.

David
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Old 31st Jul 2020, 9:52 pm   #25
squegging
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Default Re: Stereo 50w 807 PP amp.

Ah, this takes me back to my schoolboy days. Built an amp with 807's, and in a hurry to finish without any top cap connectors, I soldered the wires on. Anyway it worked quite well, the purple glow on the envelopes danced about with the music until bang! The valves got rather warm and melted the soldered top caps (well one of them anyway!) and the resulting flash ruined the OP transformer.

Keith

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Old 20th Aug 2020, 1:41 am   #26
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Default Re: Stereo 50w 807 PP amp.

Andy, are you able to measure gain and phase shift at LF with no feedback - eg. using X-Y scope display? Knowing the basic response of the output transformer (ie. with really large coupling caps to lower their influence to below the frequency response of the OPT) may then help identify what stability management technique to use such as selecting the coupling cap sizes, and what LF step network may be needed to give sufficient stability margin. The tricky aspect is that the OPT response will change with signal level - so getting a good view of that characteristic may require more testing effort.
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Old 21st Aug 2020, 8:08 am   #27
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Default Re: Stereo 50w 807 PP amp.

I'll reply over on DIY re the testing but am familiar with selected coupling caps etc . In Radio designers he says the trick is selecting C value's etc so that individual stages are either degenerative or regenerative as regards the frequencies concerned, there being different approaches for a two stage and three stage amplifier.

"The tricky aspect is that the OPT response will change with signal level - so getting a good view of that characteristic may require more testing effort." Your right there.

Thanks, Andy.
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Old 21st Aug 2020, 9:49 am   #28
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Default Re: Stereo 50w 807 PP amp.

Andy,
Do you want me to toss in a couple of spare 807's in the package for you?
(BTW- I found the basket of lost tubes!!)
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Old 31st Aug 2020, 12:46 pm   #29
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Default Re: Stereo 50w 807 PP amp.

Email sent Frank.

Made a video in regards to this amp, if interested you can see it here - https://www.youtube.com/watch?v=jEFoAJKlqDo

Andy.
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